The present invention relates generally to contaminant-blocking or deflecting labyrinth seals and, more particularly, to labyrinth seals that separate two spaces containing fluids at different pressures, such as, for example, a seal placed between a shaft and compression chamber housing of a gas turbine or of an aviation turbojet.
Environmental control systems for aircraft typically employ air cycle machines and heat exchangers to cool and condition high-pressure air supplied by either the engines or an auxiliary power unit. In these systems, a compressed air supply air is further compressed in a compressor, cooled in a heat exchanger, and expanded in a turbine. The turbine outlet air, cooled by expansion, flows into the aircraft. Since the aircraft air is maintained at a lower pressure than the supply air, properly designed systems provide conditioned air at temperatures low enough to cool both the cabin and the aircraft avionics.
In such systems, it is continuously a problem to seal an opening through which a rotatable shaft protrudes. The problem is manifested in the difficulty of preventing leakage or loss of pressure or loss of vacuum while at the same time avoiding undue shaft friction. The shaft friction can generate heat, cause loss of power, and damage the machinery.
In aviation turbojets, a seal is required to separate an upstream space, which contains air at the exhaust pressure of the last compression stage of the turbojet (e.g., 20 bar) from a downstream space which connects to the first mobile turbine blading at a lower pressure (e.g., 10 bar). Such seals are also required in space vehicle applications, in underwater vehicle applications, and in applications of ground-based turbo-machinery.
A labyrinth seal is a type of mechanical seal that is not fluid-tight but limits leakage by means of a tortuous path, and is often used to separate two spaces containing fluid at different pressures and, in particular, to seal an opening between a rotatable shaft and a journal bearing.
Air cycle machines that operate in dusty environments prematurely fail due to sand or dust entering the air bearing cooling circuit causing bearing wear, erosion and failure. At startup, an air cycle machine is not yet fully pressurized, and when a labyrinth seal that has one inner diameter is used to seal the opening through which a rotatable shaft protrudes, contaminated air flowing through the labyrinth seal flows directly into the adjacent journal bearings. Furthermore, sand and dust particles deposited between the bearings and the shaft abrade the bearings when the shaft starts to move against the stationary bearings.
FIG. 1 shows a cross-sectional partial view of an air cycle machine 10, including a prior art seal assembly 12 having only one inner diameter.
The air cycle machine 10 comprises a compressor housing 180 that admits bleed air flow from the engine (not shown). The bleed air under normal engine pressure enters the compressor inlet 186 (see FIG. 2), passes between the blades of initially stationary compressor wheel 30 that is mounted between 2 colinear shafts 20 and AAA. The engine bleed air exits the compressor wheel 30, passes through the compressor diffuser 32 into the compressor housing scroll 34, and exits into the system duct (not shown) enroute to a heat exchanger (not shown) for pre-cooling. The pre-cooled bleed air enters the turbine housing 18 that houses the turbine nozzle 14 that ejects the bleed air through the nozzle holes to drive the initially stationary turbine wheel 16 that is mounted on the same shaft 20 that mounts the compressor wheel. The air cycle machine 10 may have a thrust disk 24 mounted behind the turbine wheel 16, wherein the thrust disk 24 has a plurality of thrust bearings 28 on each side of the thrust disk 24.
A compressor wheel 30 mounted on the shaft 20 may expel compressed air into a compressor diffuser 32 opening up into an annular shaped compressor housing scroll 34 having a plurality of journal bearings 36. The plurality of journal bearings 36 foils around the shaft 20. The compressor wheel 30 may be connected to the turbine wheel 16 by the shaft 20, which may rotate about the air cycle machine centerline 40. A prior art seal assembly 12 may be disposed between the compressor wheel 30 and the journal bearing 36.
FIG. 1 further shows an exemplary air flow through the air cycle machine 10 from operation at startup when the static air pressure may be about 1 bar throughout the air cycle machine 10 as air pressures are increasing during acceleration from operation at startup to operation at rated speed.
The compressor inlet air flow 42 (indicated by multiple dashed arrows) leaks through the prior art seal assembly 12 to vented outflow 44 via the compressor flow leakage 46 (indicated by dashed arrows), so that contaminants present in the compressor inlet air flow 42 are deposited in the plurality of journal bearings 36 and the plurality of thrust bearings 28.
U.S. Pat. No. 4,320,903 to Ayache et al. discloses a labyrinth seal mounted between a shaft and the housing of a gas combustion chamber. The labyrinth seal comprises a wheel mounted on the shaft and equipped at its periphery with a series of fins, which operate in conjunction with a sealing surface made of a honeycomb material designed to be worn down by the friction of the fins acting thereon. A portion of the air taken from the housing flows through radial canals into holes comprising an overflow valve. When the turbojet is operating at full load, the overflow valve is closed, so that all the air taken in cools the seal. During a deceleration, the valve opens as the velocity decreases until it is fully open at the slow speed, thus opening to the air in the canals a more permeable passage than the one through the sealing surface. At low velocity, the totality of the air, instead of forcing its way through the sealing surface, escapes into the atmosphere through the overflow valve.
U.S. Pat. No. 5,085,443 to Richards et al. discloses an air cycle machine including a labyrinth seal having one inner diameter. When the air cycle machine starts, contaminated air that leaks through the labyrinth seal may deposit contaminants in the adjacent journal bearings.
As can be seen, there is a need for a labyrinth seal that blocks and deflects contaminated fluid so that contaminants do not flow directly into adjacent regions during startup.